Display panel has been widely used in a variety of electronic devices in modern technology, such as cellular phone, television, personal computer, and so on. With the development of Touch with Display Driver Integration (TDDI) technology, touch and display functions may be integrated in one chip to drive a touch display panel which is a display panel with integrated touch sensing capability. The main advantages of TDDI technology include cost reduction, a thinner touch display panel, and better performance.
FIG. 1 shows a timing scheme of a driving process for driving a touch display panel and a signal waveform of a touch driving signal according to the related art. The touch display panel includes a plurality of electrodes, which may be operated as display common electrodes when displaying images or as touch electrodes when touch sensing. The touch display panel alternately operates in a display phase and a touch phase. The time length of the display phase is a display period (abbreviated as DP in the following description), and the time length of the touch phase is a touch period (abbreviated as TP in the following description). Display periods and touch periods are alternately arranged. The display period may also be called as the display part, and the touch period may also be called as the touch part. A touch display period refers to a combination of a display period and a touch period adjacent to the display period.
In the display phase, an output signal OUT as a common reference voltage VCOM, such as −0.5V, is applied from a driving chip to the touch display panel, such that the voltage measured on the electrode is the common reference voltage VCOM. In the touch phase, the output signal OUT as the touch driving signal applied to the electrode (as touch electrodes) is an alternating current (AC) signal, having a high level such as 3.5V, and a low level such as 1.5V, with a direct current (DC) component 2.5V. The DC component of the output signal OUT appears as a periodic signal, having a high level 2.5V (in the touch phase) and a low level −0.5V (in the display phase),
FIG. 2 shows an example of time arrangement of display phases and touch phases for a touch display panel according to the related art. The touch display panel is assumed with Full HD resolution (1920×1080 pixels). FIG. 2 is illustrated based on a configuration of the display frame rate 60 Hz and the touch scan rate 120 Hz. One display frame period is 16.6 ms and one touch frame period is 8.3 ms. In this example, the display frame period is equivalent to two touch frame periods. That is, for every image frame to be displayed, touch sensing (or said touch scan) for the entire touch display panel is performed twice.
Refer to FIG. 2, a touch frame period is equivalent to 12 touch display periods, and a display frame is equivalent to 24 touch display periods, wherein each touch display period includes a display period and a touch period. In a display frame period, 24 display periods and 24 touch periods are interlaced or said alternately arranged. In each display period, 1920/24=80 gate lines are sequentially turned on such that data voltages of 80 horizontal display lines (80 H) are outputted line by line to the touch display panel. After a display period (denoted as 80 H), a touch period follows.
Refer to FIG. 2 again, based on the display frame rate 60 Hz and total 24 touch display periods in a display frame period, each touch display period occupies 16.6 ms/24=694 μs, where each display period for driving 80 H occupies 504 μs (if based on a case that driving a row of pixels, i.e., a horizontal display line, costs 6.3 μs) and each touch period occupies 190 μs. Refer to the waveform shown in FIG. 1, the DC component of the output signal OUT from the driving chip appears as a periodic signal having a switching frequency (which means the display phase and touch phase are periodically switching). The switching frequency fsw is thus 1/694 μs=1.44 KHz. In this example the switching frequency 1.44 KHz is within the frequency range of audible noise, which may influence user experience when using an electronic device having the touch display panel, such as a mobile phone. There are also other frequency components (harmonic tones) with frequency that is a positive integer multiple of the base switching frequency fsw, such as the second harmonic 2×fsw, the third harmonic 3×fsw, and so on. These frequency components constitute audible noise when the touch sensing function is enabled.